Ion implantation has become a standard technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material to form a region of desired conductivity.
Ion implantation systems usually include an ion source for converting a gas or solid material into a well-defined ion beam. The ion beam is mass analyzed to eliminate undesired ion species, is accelerated to a desired energy and is directed onto a target plane. The beam is distributed over the target area by beam scanning, by target movement or by a combination of beam scanning and target movement. Examples of prior art ion implanters are disclosed in U.S. Pat. No. 4,276,477 issued Jun. 30, 1981 Enge; U.S. Pat. No. 4,283,631 issued Aug. 11, 1981 to Turner; U.S. Pat. No. 4,899,059 issued Feb. 6, 1990 to Freytsis et al; and U.S. Pat. No. 4,922,106 issued May 1, 1990 Berrian et al.
A well-known trend in the semiconductor industry is toward smaller, higher speed devices. In particular, both the lateral dimensions and the depths of features in semiconductor devices are decreasing. State of the art semiconductor devices require junction depths less than 1000 angstroms and may eventually require junction depths on the order of 200 angstroms or less.
The implanted depth of the dopant material is determined, at least in part, by the energy of the ions implanted into the semiconductor wafer. Shallow junctions are obtained with low implant energies. Ion implanters are typically designed for efficient operation at relatively high implant energies, for example in the range of 50 keV to 400 keV, and may not function efficiently at the energies required for shallow junction implantation. At low implant energies, the current delivered to the wafer is much lower than desired and in some cases may be near zero. As a result, extremely long implant times are required to achieve a specified dose, and throughput is adversely affected. Such reduction in throughput increases fabrication cost and is unacceptable to semiconductor device manufacturers.
Semiconductor device manufacturers typically require the use of parallel scan techniques, wherein the ion beam has a constant angle of incidence on the semiconductor wafer. One known technique for achieving parallel scanning involves deflection of the ion beam with a magnetic or electrostatic scanner to form a fan-shaped beam. The scanner is followed by an angle corrector, which deflects the fan-shaped beam into parallel trajectories to form a scanned ion beam. The scanned ion beam is distributed over the semiconductor wafer by mechanically moving the wafer in a direction perpendicular to the plane of the scanned ion beam.
The angle corrector typically comprises a pair of magnetic polepieces spaced apart by a relatively small dimension in the direction perpendicular to the scanned ion beam. The magnetic fields produced by the angle corrector deflect the ion beam in the scanning plane to produce parallel trajectories. At low ion beam energies, the ion beam tends to expand due to the well-known space charge effect, and a portion of the ion beam may be incident on the polepieces of the angle corrector, thereby reducing the current delivered to the semiconductor wafer. In addition to the above problems associated with reduced beam current and extended implant times, contaminants may be generated through sputtering when energetic ions impinge on surfaces along the beamline. The spacing between the polepieces of the angle corrector cannot be increased without degrading the performance of the angle corrector.
Accordingly, there is a need for improved methods and apparatus for controlling an ion beam to alleviate one or more of the above-described disadvantages, without substantial impact on the size and cost of the ion implanter.